Amplifier circuits are commonplace in modern electronic design. Some desirable features for a single-ended amplifier circuit include high speed operation with a low input offset voltage. For example, the specifications of the amplifier may require operation in the Intermediate Frequency (IF) band with an input signal in the millivolt range. It is desirable to have a substantially zero input offset voltage for the amplifier with such low level input signals. Otherwise, the input signal may not reach a sufficient level to activate the output signal of the amplifier. Another consideration for an amplifier operating with small input signals is the noise level on the power supply conductors, wherein the power supply noise may feed through and overwhelm the low level input signal. Given such restrictive specifications, the prior art has typically employed linear bipolar technology to achieve the low offset voltage and power supply noise isolation since single-ended MOS amplifiers with low level input signals historically have been susceptible to noise.
In many applications, it is necessary to amplify the low level analog input signal to digital levels for further processing through digital circuits. The digital circuits are typically implemented with MOS technology because of the low power consumption while high speed linear circuits are bipolar in design. Thus, conventional practice has taught either using separate bipolar and MOS integrated circuit (IC) chips for the amplifier and digital logic functions, respectively, thereby maintaining a single technology per IC, or combining bipolar and MOS technologies onto a single application specific integrated circuit (ASIC). However, both of these options increase the overall cost as compared to a single technology design for both the linear and digital circuits on one integrated circuit.
Hence, what is needed is an improved single-ended amplifier circuit implemented in MOS technology combining high speed operation with a low offset voltage for low level input signals while providing noise isolation from the power supply conductors.